A hydrogen and oxygen proton exchange membrane fuel cell (PEMFC) is an electrochemical apparatus which directly transforms chemical energy to electrical energy. Unlike a conventional internal combustion engine, the energy conversion of the PEMFC is not confined by Carnot cycle, and has a higher theoretical energy conversion efficiency. The PEMFC produces water and no harmful emissions by using hydrogen and oxygen gases as reactants, which makes it attractive and popular in electric stations, vehicles, and mobile power sources.
The PEMFC produces a direct current with an output voltage smaller than 1 V (typically 0.7 V) per cell. A series connection of multiple PEMFC cells, which forms a PEMFC stack, achieves a higher voltage. One single PEMFC cell includes components such as gas diffusion layer (GDL) for anode, membrane electrode assemblies (MEA), and GDL for cathode.
The fuel cell power generation system comprises the PEMFC stack which is an essential member, and multiple auxiliary systems, such as air and hydrogen supplying systems, cooling system, power adjusting system, moisture adjusting system, and control system, to assist operation of the stack. The air supplying system inputs a suitable amount of oxidants, such as air, and controls the temperature, pressure, and flow rate of the air supplied. The hydrogen supplying system inputs hydrogen, and controls the pressure and flow rate of the hydrogen gas supplied. The cooling system maintains the temperature of the stack to a suitable level. The power adjusting system controls the output voltage and current of the stack to meet the needs of an electrical load. The moisture adjusting system adjusts the wetness of the air that is supplied to the stack, to be within an optimal range, neither too dry nor too wet. The control system controls each auxiliary system to achieve a best working state of the stack.
The water produced by the PEMFC as gas or liquid is expelled from cathode by an air flow. A high flow rate of the air supplied to the stack can efficiently expel water. However, when the stack has a low load, only a small amount of water is produced. A high flow rate of air may dry the proton exchange membrane, which causes degeneration in the performance of the proton exchange membrane. Yet, a relatively low flow rate of air may expel water inefficiently and cause the fuel cell to flood. Precise control of the flow rate and moisture of the air is difficult to achieve, especially for a stack which has a large amount of non-identical cells.
The working state, such as the moistness of the proton exchange membrane and the flooded or partly flooded state of the fuel cell has a relationship with equivalent circuit impedance of the fuel cell. By obtaining a measure of the equivalent circuit impedance in real time, the working state of the fuel cell can be precisely analyzed and adjusted.